The invention relates to a method and apparatus for pneumatically measuring the volume of a space or spaces in a workpiece, in particular the combustion chamber spaces of a cylinder head for an internal combustion engine. More specifically, the invention relates to improvements in arrangements of the type where the measuring apparatus measures the pressure change in the volume to be measured and by this means the volume is determined.
In practice the volumes of spaces of workpieces need to be accurately measured. This is especially so if the spaces are formed, not by machining operations, but rather by casting of the workpiece. For example, cylinder heads for internal combustion engines that have combustion spaces formed therein by means of casting manufacturing methods and which have in addition relatively irregular shaped combustion spaces present particularly difficult to measure combustion space volumes. On the other hand, however, the volume of the combustion chamber spaces in the cylinder head have a large influence on the quality of combustion and therewith the power and the fuel consumption of the engine. In order to manufacture fuel efficient motor vehicles it is in the interest of the manufacturer to be able to exactly measure the volumes of the combustion spaces in the cylinder head so that he can then maintain the closest possible tolerance on the dimensions of these combustion spaces.
It has been contemplated to measure the volumes of combustion chambers of cylinder heads by means of sealing up the valve seats and bores for the sparkplugs and then after wards filling the spaces with a pipette supplying a fluid, such as oil, and then monitoring the volume of oil required to fill such spaces. This procedure is time consuming and also inexact because it depends upon the skill and carefulness of the person carrying out the tests and how exactly he monitors the filling of the combustion spaces with the measuring fluid. Therefore, this procedure is only practically usable for prototypes or testing of single parts.
It has also been contemplated to use a procedure with pneumatic measurement of the volumes of combustion chamber spaces of the cylinder heads, which procedure can also be used with prototypes or testing of single parts. In this procedure a two-chamber differential pressure transmitter is filled with pressurized air, whereby both chambers are separated from one another. Thereafter, one chamber is connected with a sealed combustion chamber space and the other chamber with a predetermined comparison volume of predetermined size. The differential pressure transmitter then gives a signal which corresponds to the differences between the volumes of the combustion chamber spaces and the predetermined control or comparison volume. This procedure is relatively highly accurate, however, it is not practical for 100% testing of many cylinder heads, particularly, such as would be required on an assembly line, because the measured result is dependent upon the temperature of the cylinder head being measured, and this temperature can vary from one cylinder head to another. This variance in cylinder head temperatures can result from the variances in prior machining or casting operation and the variance in time lapse from a given prior manufacturing operation and the combustion space measurement.
The invention is based upon the problem of developing a procedure using the principal of pneumatic measurement of the known art, usable for 100% testing of all workpieces transported on an assembly line, even if these workpieces exhibit different temperatures. This problem is solved by the invention by providing the help of an additional pneumatic measuring apparatus connected to a test chamber with known volume and which is maintained at the temperature of the workpiece, the system being connected so that the temperature dependent pressure change is automatically compensated for in the measurement results obtained by the measuring apparatus.
The invention is based upon the idea that the test chamber of the additional pneumatic measuring apparatus has the same temperature as the spaces being measured so that temperature related pressure changes can be taken into consideration in the volume measurements
These and other objects, features, and advantages of the present invention will become more apparent from the following description when taken in connection with the accompanying drawings which show, for purposes of illustration, several preferred eabodiments in accordance with the present invention.